It is usual to protect objects against external constraints, physical as well as chemical, by means of protective coatings generally consisting in deposits of thin layers employing for instance nickel or chromium. A particularly effective coating having an attractive appearance consists of covering the object by aluminium oxide if the base substrate is of pure aluminium or an alloy thereof.
The Japanese patent publication JP-A-59-40 916 describes such a treatment for a watch case the substrate of which is formed of an alloy of aluminium. This treatment consists of anodically oxidizing the substrate in a solution of 10 -20% of sulphuric acid maintained between -4 and 10.degree. C., in which solution air is introduced in order to agitate it. The electrical potential and current density are progressively increased from 24 to 90 V and from 2 to 7 A/dm.sup.2 respectively, in a manner such that a layer of aluminium oxide is created which exhibits crystalline growths at the surface of the substrate. The oxide layer is thereafter impregnated by an aqueous liquid containing a fluorocarbon polymer having particles in the form of grains, the dimension of which is smaller than 2 .mu.m and which place themselves between the crystalline excrescences in order finally to fill up the holes between said excrescences. Thanks to this procedure, the wear resistance of the visible parts of the watch case is improved.
Apart from the interesting details which it divulges, the cited publication does not indicate that it is necessary in order to obtain a regular coating presenting the attractive satin matt aspect which belongs normally to the anodization of aluminium to practise such anodization on an aluminium substrate which is substantially pure or at least lacking elements which could spoil the fine aspect. The patent document US-A-4 640 625 puts the finger on this necessity. Effectively in this document it is shown that the watch case formed of an anodized aluminium alloy contains less than 0.01% of silicon and less than 0.01% of iron (by weight). This low value of silicon and iron effectively permits one to avoid diminution of the brilliance of the anodized layer.
In an article entitled "All That Glitters" from the "Horological Journal" No. 111, Aug. 1968, pages 22-24, it is also indicated that in order to anodize a watch case formed of aluminium, it is essential to have available a material the purity of which is 99.99% if one wishes to obtain good results.
The same observations appear in the patent documents FR-A-2 360 112 and US-E-28527 in which, for the first, the watch part is moulded in pure aluminium under pressure and, for the second, the parts are massively formed with 99.8% of pure aluminium.